Exhaust Pipe Diameter Calculator
Size exhaust tubing for your power level without falling into the bigger-is-better trap. Too large costs low end torque.
Your engine
- Sized for peak power. A street engine spends its life well below peak, which is why velocity matters more than raw area.
- Tubing is sold by outside diameter. Mandrel bent pipe keeps its full area through bends, crush bent does not.
- Header primary sizing is a separate problem and is not covered here.
Bigger is not better
An exhaust has two jobs that pull against each other. It has to let gas out without restriction at peak power, and it has to keep gas velocity high enough at low rpm that each pulse helps pull the next one out behind it.
Oversize the pipe and velocity collapses at low rpm. The engine loses the scavenging effect, and the result is a car that feels flat everywhere you actually drive it while gaining a few horsepower at an rpm you rarely see.
The numbers
Exhaust volume flow runs at roughly 2.2 CFM per horsepower on a naturally aspirated gasoline engine. Divide that by a target gas velocity and you have the area you need. Useful velocity sits between about 11,000 and 20,000 feet per minute, with street builds at the higher end.
Turbos change the rules
A turbine drops exhaust pressure and temperature substantially, so gas downstream is denser and slower. Turbo cars want noticeably larger pipe than the same power naturally aspirated, and the usual restriction penalty for going large mostly does not apply.
Frequently asked questions
What size exhaust for 400 hp?
On a naturally aspirated street V8 with true duals, 2.5 inch per side is the usual answer, which is what this calculator lands on. A single pipe carrying the same 400 hp wants around 3.5 inch.
Will a bigger exhaust add power?
Only if the current one is actually restricting flow. If it is not, a larger pipe drops gas velocity and costs low end torque while gaining little or nothing at peak. Measure backpressure before assuming the exhaust is the bottleneck.
Are true duals better than a single pipe?
Above roughly 400 hp, usually. Below about 250 hp a single well sized pipe often makes more usable power, because two pipes split the flow and neither maintains good velocity. An X or H crossover recovers much of the difference by letting the banks share.
Does exhaust diameter affect sound?
Yes. Larger pipe generally means deeper and louder, since there is less velocity and more volume for low frequencies. Pipe length and muffler design matter more than diameter for controlling drone.
Everything on this page is an estimate worked out from the numbers you entered and typical figures for the conditions you picked. Your vehicle, your parts and your conditions will differ. Check anything you intend to act on against the manufacturer's specification, the service manual, or a qualified technician. Verifying the result before relying on it is up to you.